This invention relates generally to rotation sensor which generates a signal indicative of a reference position or rotational angle of a rotary member used in various apparatus, such as recording/reproducing apparatus including video tape recorder, video disc player or the like.
In a recording/reproducing apparatus, a reference position of a rotary member, such as a turntable of a video disc or a rotary drum carrying video heads, has to be detected for performing accurate playback for instance. According to a conventional rotation sensor, a portion of a rotary disc arranged to rotate with a disc motor is magnetized and the passage of the same is detected by a coil providing to oppose the magnetized portion. However, in such an arrangement, since the number of coil turns is very small, such as one or two, in correspondence with the number of poles of the magnetized portion, the output signal level from the coil is relatively low and signal-to-noise (S/N) ratio is also relatively low. Furthermore, such rotation sensor can produce an output signal once per one revolution of a rotary member, and thus accurate detection of rotational speed is not achieved.
On the other hand, conventional tachogenerator comprises a plurality of magnetized portions, i.e. N and S poles, circumferentially arranged alternately, and a circular coil pattern associated with the magnetized portions to detect the passage of the same. However, such conventional tachogenerator produces an output signal in the form of pulses which indicate only the rotational speed but no reference position or angle.
In this way, according to conventional techniques it is required to provide a reference position detector and a tachogenerator separately for obtaining two signals respectively indicating a reference position or rotational angle and a rotational speed of the rotary member. As a result, entire structure of such separated arrangement is apt to be bulky, while output signal from the reference position detector or sensor is of relatively low amplitude. Especially, such low amplitude output results in low signal-to-noise ratio which may cause inaccurate detection of a reference position of a rotary member.
Furthermore, conventional arrangement requires a large number of parts due to such separate structure, resulting in high manufacturing cost.